The bandwidth shortage increasingly experienced by mobile carriers has motivated the exploration of the underutilized Millimeter Wave (mmWave) frequency spectrum between around 30 G and 300 G Hz for the next generation broadband cellular communication networks. The available spectrum of mmWave band is hundreds of times greater than the conventional cellular system. The mmWave wireless network uses directional communications with narrow beams and can support multi-gigabit data rate. The underutilized bandwidth of the mmWave spectrum has very small wavelengths, which enables large number of miniaturized antennas to be placed in a small area. Such miniaturized antenna system can produce high beamforming gains through electrically steerable arrays generating directional transmissions. With recent advances in mmWave semiconductor circuitry, mmWave wireless system has become a promising solution for real implementation. However, the heavy reliance on directional transmissions and the vulnerability of the propagation environment present particular challenges for the mmWave network with beamforming.
In principle, beam management mechanism, which includes both initial beam alignment and subsequent beam tracking, ensures that base station (BS) beam and user equipment (UE) beam are aligned for data communication. To ensure beam alignment, beam-tracking operation should be adapted in response to channel changes. However, in mmWave systems, transmission path lifetime is expected one order of magnitude shorter than traditional cellular bands due to wavelength difference. Combined with dedicated beam with small spatial coverage, the number of effective transmission paths for a dedicated beam could be rather limited, thus more vulnerable to UE movements and environmental changes. Beam failure recovery mechanism is designed to handle the rare case beam tracking issue, e.g., when feedback rate for beam management may not be frequent enough. For example, sudden blockage can result in lost connection and beam failure.
Radio link monitor (RLM) is designed to ensure proper link quality can be achieved from higher layer perspective, e.g., radio resource control (RRC) layer. RLM exists in single-beam systems such as LTE systems. Under RLM, periodic physical layer L1 indications on link quality, e.g., In-Sync and Out-of-Sync (IS/OOS) indications, are monitored. Radio link failure (RLF) can be declared upon consecutive OOS exceeds number NOOS and accumulative IS does not reach number NIS before the expiry of TIS timer.
For beamformed access system, both beam recovery procedure and RLM procedure are essential to link quality maintenance. Interaction between the two procedures for harmonized operation is required.